When you tell small, selfish lies, you train your brain to tell big lies

It's a game of cards. You throw down a hand, your friend throws down a better one. But he's distracted, watching a soap opera on TV.

"Who won?" he asks.

"Er, I did," you say, sweeping a small pile of chips over to your side of the table. You feel guilty, but the money is too tempting to turn down. And look! He hardly seems to care.

A few rounds later, he gets up to use the bathroom. When he comes back, he points to a stack of chips off to the side of the table. "Were those yours or mine?"

"Mine," you lie, more easily this time. On the way out the door, you stumble, knocking over a big motorcycle with a loud clang. A biker comes running around the corner.

"Who did that?"

You point to a couple halfway across the parking lot, their backs to you.

"It was them, I saw it."

The biker takes off after them, shouting and waving an insurance card. You slip into your car and flee the scene, whistling a tune.

So why were you such a jerk?

It turns out that telling small, selfish lies trains your brain to get used to lying - and leads you to tell bigger ones in the future.

A study published Monday in Nature Neuroscience found that when people tell several selfish lies in a row, their brains have weaker and weaker reactions to the dishonesty.

To figure this out, the researchers on the study had subjects play a game while inside an fMRI - a machine that can measure blood flow in the brain. It's a rough method for watching what parts of the brain become active over time, with more active parts of the brain needing more blood.

The subjects helped a partner they couldn't see guess how many pennies were in a jar. Sometimes they were told that if their partner guessed wrong, they would get a bigger reward and their partner would be given a smaller one. In other words, sometimes they were given an incentive to lie.

With each selfish lie, blood flow to the amygdala weakened. The amygdala regulates emotions like guilt in the brain.

In other words, once the subjects began lying, it took less emotional effort to tell the next lie.

At the same time, the researchers found, the subjects' lies grew bigger and bolder. They found themselves on a slippery slope into dishonesty.

There were some situations though when the effect didn't appear. When subjects were given the chance to tell selfless lies, lies that would help their partners but hurt them, one lie didn't lead to decreased blood flow to the amygdala or predict more lies in the future. And at no point in the experiment did the subjects risk getting caught or punished for lying.

So the lesson is this: Given the opportunity, people will lie for their own benefit. And once they start lying, their brains get comfortable with the dishonesty, and they become likelier to tell bigger, bolder lies in the future.